Sunderland say they do not know when midfielder Jan Kirchhoff will return, after the German tore cartilage in his knee.
The 26-year-old suffered the injury during Wednesday's Premier League defeat by Chelsea, coming off after 57 minutes.
Kirchhoff was making only his third appearance of the season, after two months out with a hamstring injury.
He is the fifth Sunderland midfielder ruled out through injury.
Lee Cattermole, Lynden Gooch, Paddy McNair and Duncan Watmore are also sidelined, with the latter out for the season with a cruciate knee ligament injury.